Method of making glazed AlN substrate with an Al2 O3 -SiO2 interfacial layer

ABSTRACT

A glazed AlN substrate includes: a sintered AlN body, a surface oxidized layer formed on the sintered AlN body, an Al 2  O 3  --SiO 2  layer formed on top of the intermediate surface oxidized layer, and a glass layer formed on top of the Al 2  O 3  --SiO 2  layer. In one embodiment, an additional SiO 2  layer is interposed between the glass layer and the Al 2  O 3  --SiO 2  layer. A method of producing the AlN substrate is also disclosed that permits firing of the glass layer at high temperatures.

BACKGROUND OF THE INVENTION

This invention relates to a glazed aluminum nitride (AlN) substratesuitable for use in applications such as thermal heads of a thermalrecording device, thin film circuits, thick film circuits, and the like.

A glazed ceramic substrate, in which the unevenness of the ceramicsubstrate is eliminated by thinning and coating glass on the surface, isused, for example, in a thermal head of a thermal recording system. Insuch a thermal recording system, a thermal head is held in close contactwith a thermally sensitive paper. By generating heat, a thermal resistorincorporated within the thermal head imprints a record on the thermallysensitive paper.

A glazed ceramic substrate is also widely used in thin-film circuitsubstrates. Because a high-frequency device is generally mounted on thesubstrate, a circuit substrate in a thin film hybrid IC must have asmooth surface. It must also have the properties of high precision, lownoise, and the like. Such a glazed ceramic substrate may further be usedin a thick-film circuit substrate to improve its properties.

By using as a substrate material a sintered AlN body having a greaterthermal conductivity, the heat radiating property of the resultantglazed AlN substrate is improved. However, when the glazing material onan Al₂ O₃ substrate is a glass, such as an oxide glass of SiO₂ --B₂ O₃Al₂ O₃ --CaO, applied and formed by firing on the sintered AlN body, theglass layer breaks easily.

This fragility is due to the large difference in the coefficient ofthermal expansion between the sintered AlN body and the glass layer. Toprevent breakage of this nature, a glass having a relatively smallcoefficient of thermal expansion that approximates the coefficient ofthermal expansion of the sintered AlN body has been proposed. However,using such a glass introduces an additional problem. A glass with asmaller coefficient of thermal expansion also has a higher softeningpoint. Such a glass thus requires firing at elevated temperatures tosecure the surface smoothness of the glass layer. Firing at elevatedtemperatures increases reactivity between the sintered AlN body and theglass layer. Bubbles generated by the resulting reaction produceunevenness, including blisters, on the surface of the glass layer,thereby adversely affecting the surface smoothness of the glass layer.

To prevent a reaction between the sintered AlN body and the glass layer,a glazed AlN substrate may be prepared by forming an SiO₂ layer on thesurface of the sintered AlN body through a surface oxidized layer. Theglass layer adheres to the SiO₂ layer, which prevents any undesirablereaction between the sintered AlN body and the glass layer.

The thickness of SiO₂ layer is increased to correspond to that of theglass layer. As the thickness of the SiO₂ layer is increased, there isan increased likelihood that shrinking cracks may occur in the SiO₂layer during the process of fire-forming. When these cracks occur, theglass component of the glass layer penetrates the sintered AlN body atthe time the glass layer is fired at an elevated temperature. The resultis a violent reaction, between the glass and the sintered AlN body, thatproduces bubbles in the glass layer. As a consequence, where a thickerglass layer is desired, it difficult to form it by firing at elevatedtemperatures without adversely affecting its surface smoothness.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aglazed AlN substrate that permits a glass layer with a high softeningpoint to be formed by firing at elevated temperatures without beingaffected by its thickness.

Briefly stated, a glazed AlN substrate includes: a sintered AlN body, asurface oxidized layer formed on the sintered AlN body, an Al₂ O₃ --SiO₂layer formed on top of the intermediate surface oxidized layer, and aglass layer formed on top of the Al₂ O₃ --SiO₂ layer. In one embodiment,an additional SiO₂ layer is interposed between the glass layer and theAl₂ O₃ --SiO₂ layer. A method of producing the AlN substrate is alsodisclosed that permits firing of the glass layer at high temperatures.

In accordance with these and other objects of the invention, a glazedAlN substrate includes: a sintered AlN body, a surface oxidized layer onthe sintered AlN body, a laminated Al₂ O₃ --SiO₂ layer on the surfaceoxidized layer, and a glass layer on the Al₂ O₃ --SiO₂ layer.

According to feature of the invention, a glazed AlN substrate includes:an sintered AlN body, a surface oxidized layer on the sintered AlN body,a laminated Al₂ O₃ --SiO₂ layer on the surface oxidized layer, a SiO₂layer formed on the Al₂ O₃ --SiO₂ layer, and a glass layer on the SiO₂layer.

According to a still further feature of the invention, a method ofproducing a glazed AlN substrate includes: preparing a sintered AlNbody; thermally oxidizing the sintered AlN body to form a surfaceoxidized layer thereon, wherein a resulting surface oxidized layer has athickness of from about 0.2 to about 20 μm, laminating an Al₂ O₃ --SiO₂layer on the surface oxidized layer, and forming a glass layer on theAl₂ O₃ --SiO₂ layer.

According to a still further feature of the invention, a method ofproducing a glazed AlN substrate includes: preparing a sintered AlNbody, thermally oxidizing the sintered AlN body to form a surfaceoxidized layer thereon, laminating an Al₂ O₃ --SiO₂ layer on the surfaceoxidized layer, forming an SiO₂ layer on the Al₂ O₃ --SiO₂ layer, andforming a glass layer on the SiO₂ layer.

The above, and other objects, features and advantages of the presentinvention will become apparent from the following description read inconjunction with the accompanying drawings, in which like referencesnumerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a first embodiment of a glazed AlNsubstrate of the present invention.

FIG. 2 is a cross-sectional view of a second embodiment of a glazed AlNsubstrate of the present invention.

FIG. 3 shows an equilibrium diagram of an Al₂ O₃ --SiO₂ layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a glazed AlN substrate employs a sintered AlN body11 has a thermal conductivity of at least 160 W/m. K and a coefficientof thermal expansion of about 4.4×10-6/°C. A main component of a surfaceoxidized layer 12 formed on a surface of sintered AlN body 11 is Al₂ O₃.Surface oxidized layer 12 is obtained, for example, by oxidizingsintered AlN body 11 through heat treatment at a temperature of fromabout 1100° to about 1500° C., in an atmosphere containing oxygen andsteam. Surface oxidized layer 12 has a thickness of 0.2 to 20 μm.

As a result, adhesion is improved between an Al₂ O₃ --SiO₂ layer 13laminated on surface oxidized layer 12 and sintered AlN body 11. An Al₂O₃ --SiO₂ layer 13 is formed on surface oxidized layer 12 by spincoating. A suspension of Al₂ O₃ --SiO₂ particles, in which Al₂ O₃ andSiO₂ particles have the same size of 0.05 to 5 μm, is used. Thesuspension is dispersed, and Al₂ O₃ --SiO₂ layer 13 is formed by firingat a temperature of from 900° to 1400° C. Al₂ O₃ --SiO₂ layer 13contains from about 20 to about 50% Al₂ O₃ by weight percent, with thebalance being substantially SiO₂., i.e., from about 50 to about 80%. Thethickness of Al₂ O₃ --SiO₂ layer 13 is from about 0.2 to about 20 μm.

As a result of the foregoing, the compacting of Al₂ O₃ --SiO₂ layer 13is promoted. At the same time, the strength of Al₂ O₃ --SiO₂ layer 13 isimproved. In addition, adhesion is improved between Al₂ O₃ --SiO₂ layer13 and surface oxidized layer 12, and a reaction is prevented between aglass layer 14 and sintered AlN body 11.

Glass layer 14, formed on Al₂ O₃ --SiO₂ layer 13, contains, by weight,30 to 70% SiO₂, 10 to 40% , Al₂ O₃, 1 to 10% B₂ O₃, and 10 to 40% of ametal oxide "MO") selected from a group of MgO, CaO, SrO, BaO, ZnO, andPbO. The thickness of glass layer 14 is 30 μm or more.

Glass layer 14 may be formed in accordance with manufacturing methodsusually used to produce glazed ceramic substrates. Examples of suitablemethods include: screen-printing a glass paste on an AlN substrate,superimposing a sheet prepared by applying glass on a plastic film on anAlN substrate, and dipping an AlN substrate into an emulsion of glass. Asubstrate onto which a glazed layer is applied, by any one of thesemethods or the like, is formed by firing at a temperature of from about1000° C. to about 1400° C. This results in the fusing of the interfacesof surface oxidized layer 12, Al₂ O₃ --SiO₂ layer 13, and glass layer 14on sintered AlN body 11.

A fusing reaction between Al₂ O₃ --SiO₂ layer 13 and glass layer 14results in close adhesion between sintered AlN body 11 and a glass layer14A. Further, penetration of the glass component of glass layer 14 intosintered AlN body 11 is prevented by an increase in the viscosity of theglass adjacent to the interface between Al₂ O₃ --SiO₂ layer 13 and glasslayer 14. Accordingly, glass layer 14 may be formed with a greaterthickness and a high softening point by firing at elevated temperatures,while providing improved surface smoothness.

A coefficient of thermal expansion of glass layer 14 can be maintainedwithin a range from about 3.4×10⁻⁶ to about 5.4×10⁻⁶ /°C. Accordingly,any difference between the coefficients of thermal expansion of glasslayer 14 and sintered AlN body 11 is reduced. This reduction helpsprevent the exfoliation of glass layer 14 or the occurrence of cracksand fractures on its surface.

Adhesion of surface oxidized layer 12 to Al₂ O₃ --SiO₂ layer 13 isinadequate when the thickness of surface oxidized layer 12 is less than0.2 μm. Conversely, when the thickness of surface oxidized layer 12 isgreater than 20 μm, heat radiation of the glazed AlN substrate isreduced.

When the content of Al₂ O₃ in Al₂ O₃ --SiO₂ layer 13 is greater than 50%by weight, and the content of SiO₂ in Al₂ O₃ --SiO₂ layer reaction,between the glass component of glass layer 14, softened by firing, andthe SiO₂ component in Al₂ O₃ --SiO₂ layer 13, to decrease. As a result,penetration of glass layer 14, while in a softened state at elevatedtemperatures, into sintered AlN body 11 is promoted. The resultingreaction between glass layer 14 and sintered AlN body 11 may causeblisters on the surface of glass layer 14.

On the other hand, when the content of Al₂ O₃ in Al₂ O₃ --SiO₂ layer 13is less than 20% by weight, and the content of SiO₂ in Al₂ O₃ --SiO₂layer 13 is greater than 80% by weight, the strength of Al₂ O₃ --SiO₂layer 13 is reduced, causing exfoliation of Al₂ O₃ --SiO₂ layer 13 fromthe glazed AlN substrate by a heat cycle test. Further, this promotesthe formation of cracks and fractures on the surface of Al₂ O₃ --SiO₂layer 13 after firing, especially when the thickness of Al₂ O₃ --SiO₂layer 13 is greater than 2 μm.

When the thickness of Al₂ O₃ --SiO₂ layer 13 is less than 0.2 μm, theamount of SiO₂ in Al₂ O₃ --SiO₂ layer 13 also decreases. This causesinsufficient fusing between Al₂ O₃ --SiO₂ layer 13 and glass layer 14,while in a softened state at elevated temperatures, leading to thepenetration of the glass component of glass layer 14 into sintered AlNbody 11. The resulting reaction between the glass component of glasslayer 14 and sintered AlN body 11 produces blisters on glass layer 14.Alternatively, when the thickness of Al₂ O₃ --SiO₂ layer 13 is greaterthan 20 μm, the heat radiation of the glazed AlN substrate decreases.

A reduction in size of Al₂ O₃ and SiO₂ particles in Al₂ O₃ --SiO₂suspension 13 to less than 0.05 μm causes the particles in thesuspension to gather together, making it impossible to obtain a uniformdispersion. Conversely, when the size of Al₂ O₃ and SiO₂ particles inAl₂ O₃ --SiO₂ layer 13 is greater than 5 μm, the sintering property isdecreased because of the large particle size, making it impossible toobtain a compact Al₂ O₃ --SiO₂ layer 13.

Furthermore, when the composition of glass layer 14 is not within theforegoing limit, the coefficient of thermal expansion of glass is lessthan 3.4×10⁻⁶ /°C. or more than 5.4×10⁻⁶ /°C. Consequently, formation ofglass layer 14 on sintered AlN body 11 causes exfoliation of glass layer14 and formation of cracks and fractures on the surface of glass layer14.

An increase in the amount of each of SiO₂, Al₂ O₃, MgO, CaO, SrO, BaO,and ZnO raises the softening point of glass. For this reason, even ifthe formation of glass layer 14 by firing is conducted at a prescribedtemperature (for example, of from about 1000° to about 1400° C.), thesurface smoothness of glass layer 14, which requires its viscous flow,cannot be achieved, because of the high viscosity of the glass. Further,an increase in the amount of PbO raises the coefficient of thermalexpansion of glass. This increase may cause the formation of cracks andfractures on the surface of glass layer 14. An increase in the ratio ofB₂ O₃ causes a phase splitting and does not permit forming a unifiedglass layer 14.

The first embodiment of the present invention is described in detailwith reference to the following examples.

EXAMPLE 1

Sintered AlN body 11 was cut into a size of 50 mm ×50 mm×0.8 mm. Cutsintered AlN body 11 was then thermally oxidized at a temperature of1300° C. to form surface oxidized layer 12 of Al₂ O₃ having a thicknessof 4.5 μm.

A suspension consisting of Al₂ O₃ particles and SiO₂ particles wasprepared. Al₂ O₃ --SiO₂ layer 13 with a thickness of 5.0 μm was formedon surface oxidized layer 12 of Al₂ O₃ by spin-coating using thesuspension and firing at a temperature of 1100° C. Table 1 shows themean sizes and content ratios of Al₂ O₃ particles and SiO₂ particles inthe aforementioned suspension.

Glass paste was then printed on Al₂ O₃ --SiO₂ layer 13 by screenprinting and fired at a temperature of 1200° C. The thickness of theglass layer after firing was from 40 to 50 μm. The chemical compositionof glass used contained, by weight, 55% SiO₂ ; 15% Al₂ O₃ ; 20% PbO; 5%B₂ O₃ ; and 5% CaO.

The conditions of the interface between glass layer 14 and Al₂ O₃ --SiO₂layer 13, and the surface property of glass layer 14, were evaluated foreach glazed AlN substrate manufactured. This evaluation was performed byobserving the cross sections and surfaces by scanning electronmicroscopy "SEM"). There was no indication that the glass component ofsoftened glass layer 14 penetrated into sintered AlN body 11 in any ofthe glazed AlN substrates, except for Structure 1-1, Structure 1-5,Structure 1-6, Structure 1-9, Structure 1-10, and Structure 1-13. Nobubbles were observed in glass layer 14. Furthermore, no unevenness,including blisters from the generation of bubbles, was found on thesurface of glass layer 14.

The surface of glass layer 14 was evaluated by a roughness gauge. Asshown in Table 1, the results indicated a low value of surface roughness(Ra) ranging from 0.05 to 0.07. Table 1 also shows the roughness of thesurface of the substrate, which was surface-treated without the glazedglass layer. The results show a low value of surface roughness (Ra),ranging from about 0.13 to about 0.20 μm.

Based upon the aforementioned analysis, the glazed AlN substrate,according to the first embodiment of the present invention, exhibitssuperior properties for use in thermal heads thin film, circuitsubstrates, thick-film circuit substrates, and the like. Surface-treatedsubstrates without the glazed glass layer can also be used in thin-filmcircuits boards, thick-film circuits boards, and similar substrates.

EXAMPLE 2

A suspension for Al₂ O₃ --SiO₂ layer 13 consisted of Al₂ O₃ --SiO₂particles composed, by weight, of 33% Al₂ O₃ ; 67% SiO₂. A glass pastewas screen-printed on Al₂ O₃ --SiO₂ layer 13 after firing. Glass layer14 was then formed by firing at a prescribed temperature. The chemicalcomposition of each glass paste and the firing temperatures are shown inTable 2. The thickness of glass layer 14 after firing is 40 to 50 μm.The thicknesses of the other layers is the same as in the first example.

In this second example, the cross-sectional condition and the surfacesmoothness (glass layer surface) for each glazed AlN substratemanufactured were evaluated. Observation by SEM showed no indicationthat the glass component of any glazed AlN substrate penetrated into Al₂O₃ --SiO₂ layer 13, surface oxidized layer 12, and the substrate ofsintered AlN body 11. Further, each glass layer 14 was found to be freeof bubbles, nor was there any unevenness on the surface, includingblisters from the generation of bubbles. Similarly, evaluation of thesurface smoothness of glass layer 14 by the surface roughness gaugeshows good results in surface roughness: (Ra)=0.05 to 0.07 μm, as shownin Table 2.

Referring now to FIG. 2, a glazed AlN substrate 10' employs the sametype of sintered AlN body 11 as in the first embodiment. Surfaceoxidized layer 12 formed on the surface of sintered AlN body 11 is alsoequivalent to that of the first embodiment.

An Al₂ O₃ --SiO₂ layer 20 is formed on surface oxidized layer 12 by spincoating using a suspension of Al₂ O₃ --SiO₂ particles. The Al₂ O₃ --SiO₂suspension contains Al₂ O₃ particles and SiO₂ particles whose sizes arethe same as in the first embodiment. They are dispersed and fired at atemperature of from 900° C. to 1400° C. Al₂ O₃ --SiO₂ layer 20 contains,by weight, from about, 20 to about 75% Al₂ O₃, with a balancesubstantially of SiO₂, i.e., from about 25 to 80% by weight. Thethickness of Al₂ O₃ --SiO₂ layer 20 is from about 0.1 to about 20 μm.The compactness of Al₂ O₃ --SiO₂ layer 20 improves its strength andprovides improved adhesion with surface oxidized layer 12. Furthermore,an undesirable reaction between glass layer 14 and sintered AlN body 11is inhibited, as described below.

An SiO₂ layer 21 is formed by a sol-gel method on Al₂ O₃ --SiO₂ layer 20and fired at a temperature of from 1000° C. to 1200° C. The thickness ofSiO₂ layer 21 is from about 0.1 to about 2 μm. Alternatively, SiO₂ layer21 may also be formed by sputtering. A glass layer 14 formed on SiO₂layer 21 is the same as that described in the first embodiment. Aspreviously described, a fusing reaction between Al₂ O₃ --SiO₂ layer 20and SiO₂ layer 21, and glass layer 14, increases adhesion between SiO₂layer 21 and glass layer 14. Thus it is possible to obtain strong, closeadhesion between sintered AlN body 11 and glass layer 14. Furthermore,as in the first embodiment, an increase in the viscosity of glassadjacent to the interface between SiO₂ layer 21 and glass layer 14prevents the penetration of the glass component of glass layer 14 intosintered AlN body 11. This makes it possible to fire glass with a highsoftening point at elevated temperatures to form glass layer 14 havingimproved surface smoothness.

When the content of Al₂ O₃ particles by weight in Al₂ O₃ --SiO₂ layer 20is greater than 75%, and the content of SiO₂ particles in Al₂ O₃ --SiO₂layer 20 is less than 25%, Al₂ O₃ --SiO₂ layer 20 after firing consistsof a mixed phase of α-alumina (corundum) and mulite, as seen in theequilibrium diagram of Al₂ O₃ --SiO₂ shown in FIG. 3. As there exists noSiO₂ (Tridymite) phase, a fusing reaction with glass layer 14 in itssoftened phase does not occur.

As a consequence, the glass component of glass layer 14 penetrates intosintered AlN body 11. The resulting reaction between the glass componentof glass layer 14 and sintered AlN body 11 promotes the formation ofblisters on the surface of glass layer 14. On the other hand, when thecontent of Al₂ O₃ particles by weight in Al₂ O₃ --SiO₂ layer 20 is lessthan 20%, and the content of SiO₂ particles in Al₂ O₃ --SiO₂ layer 20 isgreater than 80%, the strength of Al₂ O₃ --SiO₂ layer 20 is reduced.

As a result, exfoliation of Al₂ O₃ --SiO₂ layer 20 from glazed AlNfractures on the surface of Al₂ O₃ --SiO₂ layer 20 after firing mayoccur, particularly when the thickness of Al₂ O₃ --SiO₂ layer 20 exceeds2 μm.

When the thickness of Al₂ O₃ --SiO₂ layer 20 is less than 0.1 μm, thereis a simultaneous decrease in the amount of SiO₂ in Al₂ O₃ --SiO₂ layer20. A decrease in SiO₂ causes a poor fusing reaction between Al₂ O₃--SiO₂ layer 20 and glass layer 14. This leads to penetration of theglass component of glass layer 14 into sintered AlN body 11, whichcauses a reaction between the glass component of glass layer 14 andsintered AlN body 11, resulting in the formation of blisters on glasslayer 14. Conversely, when the thickness of Al₂ O₃ --SiO₂ layer 13 isgreater than 20 μm, heat radiation of the glazed AlN substratedecreases.

When the thickness of SiO₂ layer 21 is less than 0.1 μm, there is asimultaneous decrease in the amount of SiO₂ in SiO₂ layer 21. A poorfusing reaction with glass layer 14 results from the decrease in SiO₂,thereby promoting the penetration of glass component of glass layer 14while in its softened state into sintered AlN body 11. As a consequence,a reaction with sintered AlN body 11 causes the formation of blisters onthe surface of glass layer 14. Conversely, when the thickness of SiO₂layer 21 exceeds 2 μm, shrinking fractures may form on SiO₂ layer 21 inthe process of firing.

The other properties are the same as those of the first embodiment.

The second embodiment of the present invention is described in detailwith reference to the following example.

EXAMPLE

Sintered AlN body 11 was cut into a shape measuring 50 mm ×50 mm×0.8 mmand thermally oxidized at a temperature of 1300° C. to form an Al₂ O₃surface oxidized layer 12 having a thickness of 4.5 μm.

An Al₂ O₃ --SiO₂ layer 20 was then formed on Al₂ O₃ surface oxidizedlayer 12 by spin coating, using a suspension consisting of Al₂ O₃ andSiO₂ particles, and fired at a temperature of 1100° C. The mean sizesand composition of Al₂ O₃ particles and SiO₂ particles in the suspensionare shown in Table 3.

Further, a SiO₂ layer 21 was formed by a sol-gel method on Al₂ O₃ --SiO₂layer 20 and fired at a temperature of 1100° C. A glass paste wasscreen-printed on SiO₂ layer 21 after firing formation.

The glass paste was fired at a temperature of 1200° C. to form glasslayer 14 having a thickness is 40 to 50 μm. The glass used to form glasslayer 14 contained, by weight, about 55% SiO₂ ; 15% Al₂ O₃ ; 20% PbO; 5%B₂ O₃ ; 5% CaO.

The conditions of the interface between glass layer 14 and SiO₂ layer21, and the surface properties of glass layer 14, were observed by SEMfor each glazed AlN substrate 10' manufactured. There was no indicationthat the glass component of softened glass layer 14 penetrated intosintered AlN body 11 in any glazed AlN substrates produced, except forStructure 2-1, Structure 2-5, Structure 2-6, Structure 2-9, Structure2-10, and Structure 1-13. Further, no bubbles were observed in glasslayer 14, which also showed no blisters on its surface. The surfacesmoothness of glass layer 14 was further evaluated by a surfaceroughness gauge. The results indicated a low value of surface roughness(Ra), ranging from 0.04 to 0.6 μm, as shown in Table 3.

Table 3 also shows roughness of the surface of the substrate, which wassurface-treated without the glazed-glass layer. These results indicate alow value of surface roughness (Ra), ranging from about 0.07 to 0.10 μm.Glazed AlN substrate, according to the second embodiment of the presentinvention, exhibited superior properties for use in thermal heads,thin-film circuit substrates, thick-film hybrid IC circuit substrates,and the like. The surface-treated substrates without the glazed-glasslayer can be used in thin-film circuit boards, thick-film circuitboards, and similar substrates.

According to an embodiment of the present invention, the use of asintered AlN body as a substrate provides excellent heat radiatingproperties. The interposition of a surface oxidized layer as a surfacelayer of a sintered AlN body increases adhesion between an Al₂ O₃ --SiO₂layer and the sintered AlN body. Thickness of the Al₂ O₃ --SiO₂ layermay be increased without generating cracks. Further, the strength of theAl₂ O₃ --SiO₂ layer is substantially improved over prior art SiO₂layers.

Accordingly, a glass layer of greater thickness may be formed on the Al₂O₃ --SiO₂ layer from a glass having a high softening point, since firingat elevated temperatures does not produce a reaction between the glasslayer and the sintered AlN body. Superior surface smoothness is attainedin a circuit substrate suitable for use in thermal heads and the like.Even without a glazed glass layer, AlN substrates with an oxidized layerand an Al₂ O₃ --SiO₂ layer can be used for thin-film circuit boards,thick-film circuit boards, and similar substrates.

Further, according to another embodiment of the present invention, thethickness of Al₂ O₃ --SiO₂ layer may be reduced, while increasing thecontent of Al₂ O₃ and SiO₂ in the Al₂ O₃ --SiO₂ layer. As a result, therange of applications may be expanded.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

                                      TABLE 1                                     __________________________________________________________________________    Properties of Al.sub.2 O.sub.3, SiO.sub.2 and Al.sub.2 O.sub.3 --SiO.sub.2     and                                                                          evaluation results after formation of glass layer (Structure 1).              Mean particle                               Surface                           size        Content                                                                              Al.sub.2 O.sub.3 --SiO.sub.2                                                         SiO.sub.2                                                                          Existance                                                                           Surface                                                                              Roughness                         (μm)     (wt %) Thickness                                                                            Thickns                                                                            of buble                                                                            Roughnes 1:                                                                          2.w/out gl                        Structure                                                                          Al.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        Al.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        (μm)                                                                              (μm)                                                                            ( : ◯, : X)                                                             Ra (μm)                                                                           Ra (μm)                        __________________________________________________________________________    1-1  0.03                                                                              0.02                                                                             40.0                                                                              60.0                                                                             4.6    0.0  X     0.45   0.36                              1-2  0.06                                                                              0.05                                                                             40.0                                                                              60.0                                                                             4.5    0.0  ◯                                                                       0.05   0.18                              1-3  0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             4.8    0.0  ◯                                                                       0.05   0.20                              1-4  0.96                                                                              0.93                                                                             40.0                                                                              60.0                                                                             4.6    0.0  ◯                                                                       0.05   0.16                              1-5  1.95                                                                              2.15                                                                             40.0                                                                              60.0                                                                             4.7    0.0  X     0.35   0.37                              1-6  0.25                                                                              0.30                                                                             10.0                                                                              90.0                                                                             4.7    0.0  X     0.48   0.40                              1-7  0.25                                                                              0.30                                                                             20.0                                                                              80.0                                                                             4.5    0.0  ◯                                                                       0.06   0.16                              1-8  0.25                                                                              0.30                                                                             50.0                                                                              50.0                                                                             4.5    0.0  ◯                                                                       0.06   0.15                              1-9  0.25                                                                              0.30                                                                             80.0                                                                              20.0                                                                             4.6    0.0  X     0.33   0.58                              1-10 0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             0.07   0.0  X     0.55   0.45                              1-11 0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             0.5    0.0  ◯                                                                       0.06   0.15                              I-12 0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             19.8   0.0  ◯                                                                       0.07   0.13                              I-13 0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             28.9   0.0  X     0.47   0.27                              __________________________________________________________________________     ◯: negative                                                       X: positive,                                                                  Roughness 1: with glass                                                       Roughness 2: without glass                                               

                                      TABLE 2                                     __________________________________________________________________________    Chemical Composition of Glass                                                                                       Surface                                 Chemical Composition (wt. %)     Burning                                                                            Roughness                               Glass                                                                             SiO2                                                                             Al2O3                                                                             B2O3                                                                              MgO                                                                              CaO                                                                              SrO                                                                              BaO                                                                              ZnO                                                                              PbO                                                                              (°C.)                                                                       Ra (μm)                              __________________________________________________________________________    1   55.0                                                                             15.0                                                                              5.0 25.0                                                                             0.0                                                                              0.0                                                                              0.0                                                                              0.0                                                                              0.0                                                                              1200 0.05                                    2   55.0                                                                             20.0                                                                              5.0 0.0                                                                              20.0                                                                             0.0                                                                              0.0                                                                              0.0                                                                              0.0                                                                              1200 0.06                                    3   60.0                                                                             15.0                                                                              5.0 0.0                                                                              0.0                                                                              20.0                                                                             0.0                                                                              0.0                                                                              0.0                                                                              1250 0.06                                    4   48.0                                                                             13.0                                                                              5.0 0.0                                                                              0.0                                                                              0.0                                                                              30.0                                                                             0.0                                                                              0.0                                                                              1200 0.05                                    5   55.0                                                                             15.0                                                                              5.0 0.0                                                                              0.0                                                                              0.0                                                                              0.0                                                                              25.0                                                                             0.0                                                                              1250 0.07                                    6   50.0                                                                             20.0                                                                              5.0 0.0                                                                              0.0                                                                              0.0                                                                              0.0                                                                              0.0                                                                              25.0                                                                             1150 0.05                                    7   55.0                                                                             15.0                                                                              5.0 10.0                                                                             15.0                                                                             0.0                                                                              0.0                                                                              0.0                                                                              0.0                                                                              1250 0.06                                    8   55.0                                                                             15.0                                                                              5.0 0.0                                                                              15.0                                                                             0.0                                                                              10.0                                                                             0.0                                                                              0.0                                                                              1250 0.05                                    9   55.0                                                                             15.0                                                                              5.0 0.0                                                                              10.0                                                                             5.0                                                                              10.0                                                                             0.0                                                                              0.0                                                                              1250 0.06                                    __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Properties of Al.sub.2 O.sub.3, SiO.sub.2 and Al.sub.2 O.sub.3 --SiO.sub.2     and evaluation                                                               results after formation of glass layer (Structure 2).                         Mean particle                              Surface                            size        Content                                                                              Al.sub.2 O.sub.3 --SiO.sub.2                                                         SiO.sub.2                                                                         Existance                                                                           Surface                                                                              Roughnes                           (μm)     (wt %) Thickness                                                                            Thkn                                                                              of buble                                                                            Roughness 1:                                                                         2.                                 Structure                                                                          Al.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        Al.sub.2 O.sub.3                                                                  SiO.sub.2                                                                        (μm)                                                                              (μm)                                                                           ( : ◯, : X)                                                             Ra (μm)                                                                           Ra (μm)                         __________________________________________________________________________    2-1  0.25                                                                              0.30                                                                             10.0                                                                              90.0                                                                             2.5    1.5 X     0.56   0.28                               2-2  0.25                                                                              0.30                                                                             20.0                                                                              80.0                                                                             2.4    1.4 ◯                                                                       0.06   0.07                               2-3  0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             2.7    1.5 ◯                                                                       0.06   0.09                               2-4  0.25                                                                              0.30                                                                             75.0                                                                              25.0                                                                             2.4    1.5 ◯                                                                       0.05   0.09                               2-5  0.25                                                                              0.30                                                                             90.0                                                                              10.0                                                                             2.5    1.4 X     0.41   0.24                               2-6  0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             0.05   1.5 X     0.46   ).@%                               2-7  0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             0.10   1.3 ◯                                                                       0.06   0.10                               2-8  0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             19.2   1.5 ◯                                                                       0.06   0.08                               2-9  0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             25.6   1.4 X     0.38   0.22                               2-10 0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             2.6    0.08                                                                              X     0.54   0.21                               2-11 0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             2.4    0.11                                                                              ◯                                                                       0.04   0.08                               2-12 0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             2.4    1.9 ◯                                                                       0.06   0.10                               2-13 0.25                                                                              0.30                                                                             40.0                                                                              60.0                                                                             2.3    3.8 X     0.44   0.24                               __________________________________________________________________________     ◯: negative,                                                      X: positive,                                                                  Roughness 2: without glass                                               

What is claimed is:
 1. A method for producing a glazed AlN substrate,comprising:preparing an AlN sintered body; thermally oxidizing said AlNsintered body to form a surface oxidized layer thereon, wherein saidsurface oxidized layer ranges in thickness from 0.2 to 20 μm; forming anAl₂ O₃ --SiO₂ layer on said surface oxidized layer; said Al₂ O₃ --SiO₂layer includes from about 20 to about 50% Al₂ O₃ weight percent; saidAl₂ O₃ --SiO₂ layer includes from about 80 to about 50% SiO₂ weightpercent; andforming a glass layer on said Al₂ O₃ --SiO₂ layer.
 2. Themethod for producing a glazed AlN substrate according to claim 1,wherein:said step of thermally oxidizing said AlN sintered body to forma surface oxidized layer includes heat treating said AlN sintered bodyat a temperature sufficient to oxidize said AlN sintered body in anatmosphere of oxygen and steam.
 3. The method for producing a glazed AlNsubstrate according to claim 2, wherein:said temperature ranges fromabout 1100° to 1500° C.
 4. The method for producing a glazed AlNsubstrate according to claim 1, wherein:said step of forming an Al₂ O₃--SiO₂ layer on said surface oxidized layer includes dispersing asuspension of Al₂ O₃ and SiO₂ particles; and firing said suspension at atemperature of from 900° to 1400° C.
 5. The method for producing aglazed AlN substrate according to claim 4, wherein:said Al₂ O₃ and SiO₂particles each include particle sizes of from 0.05 to 5 μm.
 6. Themethod for producing a glazed AlN substrate according to claim 1,wherein:said step of forming a glass layer includes forming said glassto a thickness of at least 30 μm.
 7. The method for producing a glazedAlN substrate according to claim 1, wherein:said glass layer includesabout 30 to 70 weight percent SiO₂ ; from 10 to 40 weight percent Al₂ O₃; from 1 to 10 weight percent B₂ O₃ ; from 10 to 40 weight percent of ametal oxide; and said metal oxide includes at least one member selectedfrom the group consisting of MgO, CaO, SrO, BaO, ZnO, and PbO.
 8. Themethod for producing a glazed AlN substrate according to claim 1,wherein:said glass layer includes a coefficient of thermal expansion offrom about 3.4×10⁻⁶ about 5.4×10⁻⁶ /°C.
 9. The method for producing aglazed AlN substrate according to claim 1, wherein:said step ofpreparing an AlN sintered body includes preparing an AlN sintered bodyhaving a thermal conductivity of at least 160 W/m.K; and a coefficientof thermal expansion of 4.4×10⁻⁶ /°C.
 10. A method for producing aglazed AlN substrate, comprising:preparing an AlN sintered body;thermally oxidizing said AlN sintered body to form a surface oxidizedlayer thereon; wherein said surface oxidized layer ranges in thicknessfrom 0.2 to 20 μm; laminating forming an Al₂ O₃ --SiO₂ layer on saidsurface oxidized layer; said Al₂ O₃ --SiO₂ layer includes from about 20to about 75% Al₂ O₃ weight percent; said Al₂ O₃ --SiO₂ layer includesfrom about 80 to about 25% SiO₂ weight percent; forming an SiO₂ layer onsaid Al₂ O₃ --SiO₂ layer; and forming a glass layer on said SiO₂ layer.11. The method for producing a glazed AlN substrate according to claim10, wherein:said step of thermally oxidizing said AlN sintered body toform a surface oxidized layer includes heat treating said AlN sinteredbody at a temperature sufficient to oxidize said AlN sintered body in anatmosphere of oxygen and steam.
 12. The method for producing a glazedAlN substrate according to claim 11, wherein:said temperature rangesfrom 1100° to 1500° C.
 13. The method for producing a glazed AlNsubstrate according to claim 10, wherein:said step of forming an Al₂ O₃--SiO₂ layer on said surface oxidized layer includes dispersing asuspension of Al₂ O₃ and SiO₂ particles; and firing said suspension at atemperature of from 900° to 1400° C.
 14. The method for producing aglazed AlN substrate according to claim 13, wherein:said Al₂ O₃ --SiO₂particles include particle sizes of 0.05 to 5 μm.
 15. The method forproducing a glazed AlN substrate according to claim 10, wherein:saidstep of forming a glass layer includes forming said glass to a thicknessof at least 30 μm.
 16. The method for producing a glazed AlN substrateaccording to claim 10, wherein:said glass layer includes from 30 to 70percent by weight SiO₂ , 10 to 40 percent by weight Al₂ O₃, 1, to 10percent by weight B₂ O₃, 10 to 40 percent by weight of a metal oxide;and said metal oxide includes at least one selected from the groupconsisting of MgO, CaO, SrO, BaO, ZnO, and PbO.
 17. The method forproducing a glazed AlN substrate according to claim 10, wherein:saidglass layer includes a coefficent of thermal expansion of from about3.4×10⁻⁶ to about 5.4×10⁻⁶ /°C.
 18. The method for producing a glazedAlN substrate according to claim 10, wherein:said step of preparing anAlN sintered body includes preparing an AlN sintered body having athermal conductivity of at least 160 W/m.K; and a coefficient of thermalexpansion of 4.4×10⁻⁶ /°C.